See also XJ6 original exterior lighting.


All the old incandescent bulbs will be replaced with much more efficient LEDs. But what is the optimal bulb for each lighting function?


Bayonet mount (Courtesy of Wikipedia)

Common LED base (socket) options

British automotive bulb sockets or bases (suitable for both incandescent and LED bulbs) are generally bayonet style, so the product identifier begins with either “BA” or “BAY”. The difference between BA and BAY is the configuration of the bulb holding pins.

The diameter of the socket is measured in millimeters, either the standard “15” or a mini “9” size. Finally, a d or s designation indicates whether the bulb is single purpose — that is, only goes on or off — or dual purpose — that is, output varies by function like a parking/brake light combination.

For example, BA15d indicates a 15mm bayonet socket designed for a dual purpose bulb while BAY9s defines a mini 9mm bayonet single purpose (with different pin locations than a BA9s).

The BA15s socket and bulb is also referred to as 1156, and the BA15d as 1142 (BAY15d is 1157). While BAY9s and BA9s are commonly used for smaller marker lights, the dual purpose mini BAY9d and BA9d are rarely deployed in British classic cars.

LED festoon design with heat sink

Festoon style bulbs are used in narrow spaces like the reverse and license plate light fixtures where a limited beam efficiently focuses output without waste. Many high output LED festoons deploy rear heat sinks to dissipate heat.

Color and brightness

After determining the proper size and pin configuration, the next issue is brightness. A typical automotive indicator light outputs about 300-450 lumens. Much under that level is too dim and presents a safety hazard, especially in bright sunlight. But excessive lumens create more heat that risks bulb failure and may also cause glare. Running lights are usually about a third of the brightness of turn and brake indicator bulbs.

Another consideration is the color of the light lens. If the lens is clear, then close to all the LED output will pass through; thus, a 400 lumen white LED appears with 400 lumen brightness. But if the lens is colored — red or amber — some of the light energy is absorbed.

Red lenses (for example, brake lights and rear side markers) filter out all light except red, so only about 25% of the filament’s total full spectrum output makes it through the red lens. An amber lens (for example, turn signals and front side markers) filters out 40% of the full spectrum, letting 60% of the total output through. Therefore, a 400 lumen white light shining through a red lens is the equivalent of a 100 lumen red light where all the output passes through.

Beam angle

A final consideration is beam angle. If the bulb is surrounded in a bowl-like housing, a complete 360° output is appropriate and no light is wasted. However, some bulbs are mounted in a coffin-like housing, like the license plate lights, where a limited 150-180° angle is optimal to reduce heat and project more light in the desired direction.

XJ6 vintage lighting equipment

The 1972 Jaguar XJ6 Operation, Maintenance and Service Handbook outlines these factory specifications for exterior Lucas lighting bulbs.

1972 XJ6 OEM Bulb Specifications
Lamp Lens color Lumens Lucas # Base Length
front side marker amber 90 989 BA9s 1.0"
front turn/running dual † amber 90/315 380 BA15d 2.2"
front turn ‡ amber 315 382 BA15s 2.2"
front running ‡ amber 90 989 BA9s 2.2"
rear running/brake dual red ∐ 90/315 380 BAY15d 2.2"
license plate clear 90 989 BA9s 1.0"
reverse clear or amber 315 273 festoon 1.7"
rear side marker red 90 989 BA9s 1.0"
rear turn indicator amber (red) ∐ 315 382 BA15s 2.2"

Footnotes: †special indirect exports used a dual purpose bulb. ‡USA direct exports used two bulbs in the front, one for turn signals and the other for running lights. ∐Standard USA export rear lenses are amber at the top for turn signals and red at the bottom for brake and running lights while non-UK domestic special exports shipped with all amber rear lenses.

Upgrade pathway

Compared to modern sedans, the XJ6 incandescent indicator lights are rather dim and it makes sense to boost brightness by 33% up to 420 lumens.

Combining lumen requirements, color, bulb purpose, and beam angle yields these optimal requirements.

Optimal LED Upgrade Requirements
Lamp Bulb color Lumens Beam Angle Max Length
front side marker amber 100 360° 1.2"
front running amber 100 360° 1.2"
front turn indicator amber 420 360° 2.2"
rear running/brake dual red 100/420 360° 2.2"
rear turn indicator amber † 420 360° 2.2"
license plate white blub through clear lens 100 150-180° 2.2"
reverse white bulb through amber lens ‡ 700 150-180° 2.2"
rear side marker red 100 360° 1.2"

Footnotes: †If dual color red/amber lens is installed. ‡Clear reverse lenses are no longer available; aftermarket replacement is amber colored.

Some of the original light assemblies have suffered decades of road debris and replacing the socket is the only way to restore the housing back to factory-new condition.

Finding the optimal LED bulbs given the vintage Jaguar sockets is a significant sourcing challenge, so one option is to replace the existing base socket so that it fits the optimal bulb and, as a side benefit, refreshes the wiring pigtails.

Front turn/running

Weathered front pigtail viewed from wheel well

Front fixture with dual purpose bulb and blocked hole

Our XJ6 was fitted for special export from the U.K. back in 1972, so the front turn and running light fixture has a dual purpose bulb that was generally used for European exports, but not in the U.S. In contrast, USA direct exports used two bulbs in the front, one for turn signals and the other for running lights.

The front fixture can be retrofitted for the standard USA two-bulb configuration by adding a new socket in the currently blocked hole. In either case, we need amber color, 360° beams, and 420 (turn)/100 (running) lumens. The advantage of adding a second bulb is that both front and rear turn indicators could use the exact same BA15s LED; 1156-A27-T from meets all the requirements. The second running bulb (100 lumens) would be BA9S-AHP5, a BA9s mini bayonet.

Sockets are either snap-in mounts that are project through the back of the light fixture, or have tabs that are soldered or riveted to the fixture. Tab mounts fully extend the socket body into the fixture space. A snap-in style projects the socket body through the back end of the housing so that bulb is partially recessed into the housing.

Single wire pigtail Cole Hersee BA9 snap-in socket 2505

If grounded immediately to the light frame, only a one-wire pigtail is required, otherwise the pigtail needs two wires for remote grounding of single bulbs.

The XJ6 front housing can handle new BA9 sockets with snap-in mounts through the 5/8″ diameter blocked hole. The existing BA15 socket will fit a single purpose BA15s LED (1156-A27-T) for the turn indicator.

Side markers

Side marker with mini BA9s bulb

BA9S-RHP5 (red)

The front and rear side markers use the same size bulb, a BA9s, with amber lenses in the front and red in the rear.

It would be possible to install new BA9 mini bayonet sockets — the old ones can be drilled out — but all four markers survived in reasonably good shape so socket surgery isn’t necessary.

The optimal LED would be about 100 lumens with a 360° beam, 1.0″ length available in both red and amber colors.

Fortunately, such a product exists: BA9S-RHP5 (red) and BA9S-AHP5 (amber) from These LEDs provide 96 lumens using a 360° beam in a 1.2″ length.

License plate

The license plate housing is badly corroded so the sockets must be replaced.

Badly aged license plate light with broken lens

After blasting, the light tray was pockmarked with rust holes, so a new floor of fiberglass resin was added. After an application of chrome paint, the part almost looks new.

Treated license light tray

There is room to increase from 9mm to 15mm bayonets or switch to festoon sockets if necessary to give us more sourcing flexibility. The optimal LED would be 100 lumens using a narrow beam less than 200° with white light; length isn’t critical.

Festoon 3910-CW30-CB (white)

The festoon style LED is best for limited beam angle requirements like the license plate housing needs. A 39mm (1.5″) festoon LED, 3910-CW30-CB from, provides 105 lumens (a cool white color) at a narrow 150° angle. New festoon sockets replace the original bulb housings.

License tray retrofitted with dual festoon LEDs

Aftermarket license plate lenses are no longer available. A 3D printed clear plastic lens adds a customized touch (click to download a PDF drawing). A STL file, readable by 3D printers, can be downloaded here.

Embossed “Kriss Motors” license plate lens

The printed 3D lens, even when fabricated with “clear” plastic, has a translucent white appearance that hides the wiring and LEDs bulbs well.

Custom 3D printed translucent lents

When lit, the license plate light provides good illumination.

Restored license plate light in action

Rear light

Top: turn; bottom: brake/running

The rear XJ6 light has a triple purpose: running, brake, and turn indicator lights. Our original lens is all red (due to our special export model), but aftermarket two-color red/amber lens are available. A red color is required at high 420 lumen output for the brake indicator and about 100 lumens for the running light with a 360° beam. Length isn’t critical.

A BAY15d bulb which fits the existing socket, 1157-R26-CBT, meets all the requirements: 420/100 lumens and a full viewing angle in a 2.1″ length.

If necessary, the BAY15d socket could be replaced. The BA15s LED 1156-A27-T from would replace the incandescent turn indicator bulb.

Reverse light

Reverse light fixture

Restoring the reverse light presents a challenge. First, our clear lenses have become pitted with age. However, only amber is now available so the optimal LED should have amber colored output.

Second, as in the license plate fixture, a festoon design provides the best narrow beam angle that can project out from a flat rear backing.

Although the depth of the reverse housing is limited, the 3.25″ length permits any size LED, and the 7/8″ width is generous enough for all festoon designs. It probably isn’t practical to substitute a BA15s socket.

Earthland L2835-48SMD high output 41mm festoon

There are no high lumen output amber festoon LEDs available. The best option is to use a white light and adjust the lumens up to compensate for the absorption loss of the amber lens. With a 40% loss due to the amber lens, a white 700 lumen LED would shine like an amber 420 lumen LED.

Several Chinese festoon LEDs claim very high lumen output — around 700-800 — for example, Earthland’s L2835-48SMD that have 48 3014 chipsets. All 48 chipsets are aligned on one flat surface, so the LED emits only in a 180° viewing angle.

The Jaguar OEM incandescent festoon bulbs are 44mm long, so a slightly smaller 41mm festoon fits comfortably. The Earthland LED rectangular chipset 0.63″ wide platform can be accommodated in the existing 0.88″ wide opening.

Fixture upgrade summary

SKUs correspond to (SBL) part numbers unless noted otherwise.

Part Socket SKU Qty Source
turn indicator bulb BA15 1156-A27-T 2 SBL
turn indicator pigtail BA15s Pico 5423PT 2 Amazon
running socket BA9 Cole Hersee 2506 2 Partdeal

BA15 pigtails with boot replacement but sockets still in good condition. Running socket is 2-wire snap in.

Part Socket SKU Qty Source
front marker bulb BA9 BA9S-AHP5 2 SBL
front marker pigtail BA9 Cole Hersee 2561 2 Partdeal
rear marker bulb BA9 BA9S-RHP5 2 SBL
rear marker pigtail BA9 Cole Hersee 2561 2 Partdeal

BA9 pigtails replacement since sockets still in good condition.

Part Socket SKU Qty Source
LED bulb festoon 3910-CW30-CB 2 SBL
socket 42mm festoon socket FS-42 2 SBL

LED bulb is 39mm and socket length is 42mm.

Part Socket SKU Qty Source
turn indicator bulb BA15 1156-A27-T 2 SBL
turn indicator pigtail BA15s Pico 5423PT 2 Amazon
brake/running bulb BAY15 1157-R26-CBT 2 SBL
brake/running pigtail BAY15d Pico 5424PT 2 Amazon

BA15 pigtails with boot replacement but sockets still in good condition.

Part Socket SKU Qty Source
LED bulb festoon L2835-48SMD 2 Amazon
socket 42mm festoon socket FS-42 2 SBL

LED bulb is 42mm and socket length is 42mm.